A conventional hydraulic power steering system includes a steering gear having a hydraulic motor. A fluid pump draws hydraulic fluid from a fluid reservoir and supplies the hydraulic fluid to the steering gear. Typically, the engine of the vehicle powers the pump to supply hydraulic fluid from the fluid reservoir to the steering gear. The steering gear includes a rotary control valve, which has a relatively rotatable valve core and valve sleeve. The rotary control valve directs hydraulic fluid to the hydraulic motor in response to steering inputs generated by turning of the steering wheel. The hydraulic motor is operatively connected to the steerable wheels of the vehicle and, when actuated, helps to turn the steerable wheels.
In a typical hydraulic power steering system, hydraulic fluid is circulated between the pump, the control valve, and the fluid reservoir. For example, U.S. Patent Application Publication No. 20040188169 discloses an apparatus for helping to turn steerable wheels of a vehicle. The apparatus includes a hydraulic power steering gear having a control valve and an electric motor that actuates the control valve. A pump supplies the control valve of the steering gear with hydraulic fluid. A vehicle speed sensor senses vehicle speed and provides a vehicle speed signal. A controller is responsive to the vehicle speed signal for controlling the pump. The controller activates the pump to supply hydraulic fluid to the steering gear when the vehicle speed signal indicates a vehicle speed below a predetermined value. The controller deactivates the pump when the vehicle speed signal indicates a vehicle speed above the predetermined value.